MOSFETs and particularly so-called power MOSFETs can be used as switches for coupling an electrical load to a power supply. For example a two-pole load can be coupled to ground by a first MOSFET and to a voltage or current source by a second MOSFET. If both of the MOSFETs are switched to their conducting state, the voltage is applied to the load thus enabling a current flow through the load. In another example the load may have four or more connection terminals for power supply, such that four or more MOSFETs are used to connect the load to a power supply. Each power supply or ground terminal of the load is coupled by one MOSFET to a power supply or ground respectively.
In conventional systems the MOSFETs may be used not only for supplying a load with DC current, but also for supplying a connected load with switched current. For example, in case the load is an AC-motor and the power supply is a direct current (DC) system, for example, a battery, then the MOSFETs can be controlled to provide alternating current as needed.
In this way MOSFETs are used conventionally in vehicles as switches for coupling a load to a power supply, wherein the power supply can be considered a battery. For example, in vehicles a battery and a generator powered by the vehicle's engine are used to supply the electrical system in the vehicle. Accordingly the electrical system is a direct current system and MOSFETs controlled by controller circuits are used as switches, wherein the MOSFETs may be arranged in pairs or in a bridge configuration using four or six or even more MOSFETs. An arrangement of MOSFETs for supplying the load conventionally is controlled by a corresponding control circuit, which may be implemented as an integrated circuit. For controlling the MOSFETs a comparatively small current is needed, such that all elements of a control circuit may be integrated into a single IC. The control circuit in turn may be coupled to another control circuit for receiving signals to switch the load on or off.
Each MOSFET comprises an intrinsic diode. Particularly the most common used N-MOSFETs comprise an intrinsic anti-parallel diode. If for any reason a voltage of inverted polarity is applied to a switched-off N-MOSFET, i.e., when there is no bias voltage applied to the gate of the N-MOSFET, the intrinsic diode will become conductive if the threshold voltage of the intrinsic diode is exceed. The current flowing through the intrinsic diode in connection with the voltage across the diode may result in a non-negligible power consumption, which heats up the MOSFET and may finally destroy the MOSFET. Particularly the intrinsic diode becomes more conductive with increasing temperature, such that a higher temperature builds up a higher current through the intrinsic diode which in turn affects a higher current. As a result one path of a bridge configuration may attract nearly the entire current thus destroying the MOSFETs in this path quickly.
In conventional systems several provisions have been developed to prevent the MOSFETs from being destroyed in case of inverted polarity. In a conventional solution an additional MOSFET is arranged in the power supply line, wherein the MOSFET is arranged such that its intrinsic diode is opposite to that of the MOSFET to protect. The additional MOSFET is switched on in normal operation and prevents a current flow through the MOSFET to protect in case of inverted polarity. In an alternative provision at least one diode is integrated into the power supply line preventing a current flow through the intrinsic diodes of the MOSFETs to protect.
However, a diode in the supply line causes a significant energy loss during normal operation. A solution comprising a MOSFET in a supply line at least needs some circuitry for controlling the MOSFET. Furthermore such a MOSFET, which shall protect power MOSFETs, must be designed as a power MOSFET itself, which makes it costly. Accordingly there is a need for an alternative solution.